WATCH: Celebs Support Ban On Chimpanzee Testing

Just over a year ago, Woody Harrelson sent a letter on behalf of PETA urging officials at the National Institutes of Health (NIH) to end a plan that would have forced 14 retired chimpanzees back into a medical testing lab. In December, the NIH took a first step towards a total ban on the practice by suspending "all new grants for biomedical and behavioral research on chimpanzees."

Today, Harrelson is once again sending out letters -- this time in support of legislation that would permanently end the suffering of these great apes. In a letter to Sen. Barbara Boxer of California, who chairs of the Senate Committee on Environment and Public Works, the 50-year-old actor pleads for her support on the Great Ape Protection and Cost Savings Act.

"Despite international criticism, the United States remains the only nation in the industrialized world that continues to conduct invasive experiments on chimpanzees, humansâ closest living genetic relatives," he writes. "Iâm writing to you today, as a constituent, to ask you to support the Great Ape Protection and Cost Savings Act (S. 810), which would permanently end the use of chimpanzees in invasive experiments and retire all federally owned chimpanzees to sanctuaries, all while saving taxpayers an estimated $30 million every year.

Harrelson goes on to say that nearly 1,000 chimps are used in experiments across the U.S., some for more than 50 years.

He adds: "This hellish experience leaves lifelong emotional scars on chimpanzees, and many of them resort to self-mutilation or suffer from depression and other psychological disorders for years after experiencing the trauma of having their minds and bodies violated."